In recent years, the semiconductor technology is rapidly developed, and the demand quantity for tantalum in sputtered films is gradually increased. In integrated circuits, tantalum, as a diffusion barrier layer, is disposed between a silicon material and a copper conductor Methods for producing a tantalum sputtering target include an ingot metallurgy (I/M) method and a powder metallurgy (P/M) method. A tantalum target which is for less demanding applications is generally prepared from a tantalum ingot. However, in some cases with higher requirements, the I/M method cannot be used, and only the powder metallurgy method can be used for producing these tantalum targets. For example, the I/M method cannot produce a tantalum-silicon alloy target for the reasons of different melting points of tantalum and silicon and a low toughness of silicon compounds.
The performance of target can directly affect the performance of sputtered film. During the formation of the film, substances which can pollute semiconductor devices cannot exist. When the sputtered film is formed, if impurities are present in a tantalum (alloys, compounds) target, the impurities will be introduced into the sputtering chamber. The introduced impurities can lead to the attachment of coarse particles onto no so that short-circuit may occur in the resultant film loop. At the same time, the impurities will become the reason for the increase of projection particles in the film. In particular, impurities, including gaseous oxygen, carbon, hydrogen, and nitrogen, present in the target will be more harmful since they can cause abnormal discharge, and thus there is a defect regarding to the homogeneity of the formed film. In addition, as to the powder metallurgy method, the homogeneity of deposited film is a function of the size of grains in the target. That is, the finer the grain in the target, the more uniform the resultant film. Therefore, there is the need for high quality tantalum powder and tantalum target in existing techniques.
Therefore, in order to obtain high quality tantalum powder and tantalum target, impurities in the tantalum powder should be reduced firstly, so as to increase the purity of the tantalum powder. However, it is well known that although the performance of metal tantalum is relatively stable, metal tantalum powder having a low particle size is more active, and it can be reacted with oxygen, and nitrogen at normal temperature, thereby to increase the contents of oxygen, nitrogen impurities in the tantalum powder. Although some metallic tantalum products, e.g., some commercially available tantalum ingots, can have a purity of up to 99.995%, or even higher, the finer tantalum powder will lead to a higher activity, and its ability to absorb oxygen, nitrogen, hydrogen, carbon is increased accordingly. Hence, as always regarded, it is relatively difficult and hard to increase the purity of tantalum powder to be 99.99% or above, and as even regarded, it is difficult to further reduce one of harmful oxygen, carbon, hydrogen and nitrogen impurities, let alone to reduce the four harmful impurities simultaneously.
Secondly, it is necessary to reduce the particle size of tantalum powder for increasing the quality of tantalum powder and tantalum target. A high purity tantalum powder having an average particle size of D50<25 μm is desired in the art.
Many skilled artisans carry out extensive research to attempt to obtain tantalum powder having a high purity and a low particle size, whereas the resultant results are not ideal.
For example, Chinese patent CN101182602A discloses a medical tantalum powder, characterized in that the oxygen content of the tantalum powder is smaller than or equal to 1500 ppm, and the nitrogen content is lower than 200 ppm. However, the content of metallic impurities and hydrogen in the powder may be high, and the particles are coarse, having a particle size D50 of about 70 μm.
Chinese Patent CN102909365 discloses a medical tantalum powder. The oxygen content of the tantalum powder is smaller than or equal to 1000 ppm, the granularity of 95% of the tantalum powder is 1.0-50.0 μm. However, by way of the simultaneous deoxygenation and dehydrogenation, since low temperature cannot effectively remove hydrogen in the tantalum powder, when the dehydrogenation and the deoxygenation are performed simultaneously, the processing temperature will be high. Moreover, tantalum powder before the deoxygenation and dehydrogenation is not subjected to a high temperature treatment, and thus its activity is higher, so that magnesium or magnesium oxide particles are easily encapsulated in the interior of tantalum particles. Thus, the magnesium or magnesium oxide particles are not easily removed during subsequent pickling process, thereby to result in high magnesium content in final product. Furthermore, in the invention, after the pickling, no heat treatment is conducted, and thus residual metal magnesium after the deoxygenation, and H, F impurities entrained during the pickling in the final tantalum powder cannot be removed. Therefore, with this method, it is difficult to reach the hydrogen content of less than 20 ppm, and the magnesium content of less than 5 ppm. It is reported that the highest purity obtained by this method may be 99.9%.
China Patent CN103447544A discloses a preparation method of particle size distribution concentrated and controllable high-purity tantalum powder, characterized in that the method includes hydrogenating high-purity tantalum ingots into tantalum scraps, subjecting the tantalum scraps to crushing and classification in turn, and then subjecting the classified tantalum powder to a low-temperature vacuum drying and a dehydrogenation treatment in turn, wherein in at least crushing and classification processes, all appliances in contact with the tantalum powder are made of tantalum with a purity more than 99.99%. The disadvantages of the method reside in that: 1. because devices in use are made of high purity tantalum, there is a high requirement to the devices, thus the corresponding cost being high; 2, because the method is lack of the deoxygenation step, the oxygen content of the resultant product is variable, being markedly different from each other, and thus it is difficult that all the oxygen contents are lower than 1000 ppm; 3. because of the use of the classification treatment, the utilization rate of materials is greatly reduced, and the refinement of the particle size of tantalum powder is difficult.
At present, the production process for metallurgical-grade tantalum powder is generally performed by way of simultaneous dehydrogenation and deoxygenation, and this will lead to limitations to designed process parameters. In particular, if a too low temperature is set, the low temperature will result in incomplete dehydrogenation and a too high hydrogen content in the final product, and at the same time, variations on the properties (such as lattice parameter, resistance, hardness, etc.) of tantalum after hydrogen absorption are not completely eliminated. If a too high temperature is set, the hydrogen gas can be fully released, whereas the too high temperature will result in that the sintered tantalum particles will grow up and that magnesium or magnesium oxide particles are encapsulated in the interior of tantalum particle so that magnesium are difficult to be removed in a subsequent pickling process, thereby resulting in a poor controllability of the particle size. That is, it is very difficult to achieve that while the oxygen content is lower than 1000 ppm, the requirement of the average particle size D50<25 μm can be assured. More unfortunately, the too high temperature will lead to a too high magnesium content. Further, in current processes, after deoxygenation and dehydrogenation, tantalum powder is subjected to pickling, baking to dry, and sieving to give the final product, without any subsequent treatments, which can result in that residual metal magnesium after the deoxygenation, and H, F impurities entrained during the pickling cannot be removed. Thus, the contents of magnesium, hydrogen and the like in the final product are too high.
Clearly, existing technologies can hardly meet retirements on the sputtered film in semiconductor technology.
Directed to the defects present in the above methods, the present invention is proposed.